In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photo lithographic processes as well as high resolution inspection instruments. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
The trend toward higher device densities in the manufacture of semiconductor devices also requires higher resolution scanning and inspection instruments for analyzing various features of semiconductor devices. A measuring apparatus is required to inspect semiconductor devices in association with manufacturing production line quality control applications as well as with product research and development. The ability to scan and/or view particular features in a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc. Accordingly, various inspection tools, such as those commercially available from KLA-Tencor, Orbot, and Inspex, have been developed to map and record wafer surface features and defects.
Defects may occur at various steps in the process of manufacturing a semiconductor product. In the development and testing of new semiconductor processing methods, it is desirable to determine which step or steps in the process are causing defects. Defect detection is also important in identifying tool cleanliness problems and trends. For example, when a silicon wafer is coated with a radiation-sensitive film or photoresist in a wafer track, particulate matter may be deposited on the wafer due to an unclean coating environment. In addition to particle defects, chemical defects are problematic and may be caused by improper ingredients or mixture ratios in the photoresist. The interaction of such chemically defective photoresist with the wafer may leave a residue on the coated wafer surface which may be detected using optical inspection tools. In similar fashion, the photoresist development process may also introduce defects into a wafer. These may include particle as well as chemistry defects.
Defects in a semiconductor wafer may be spatially repeatable. For example, a pattern defect may be caused by a mark or blemish in the mask used in a patterning step. This may cause a defect that is locationally repeatable, whereby the defect may occur at the same location in wafers patterned using the blemished mask. Similarly, dirt or marks on an exposing tool lens may cause such locationally repeatable defects in a plurality of wafers processed by the tool. Other defects may be regionally repeatable. For example, a photoresist deposition step may be performed while a wafer is rotated. Defects occurring in such a processing step may be radially repeatable, whereby defects occur according to a probability distribution in a region radially spaced from the center of the wafer by a certain distance or range. Thus, although individual defects in individual wafers may be somewhat randomly located in such a region, the region in which such defects occur may be repeatable.
Where a semiconductor wafer has been processed according to a multi-step procedure, it is desirable to identify defects associated with individual processing steps, in order to make appropriate corrections and adjustments to the process and/or to specific processing tools. Particle defects, as well as chemical residue defects may be identified in a wafer using optical measuring instrumentation and systems, such as optical microscopes. However, it may be difficult to determine the process step at which a particular defect, whether particulate or chemical, was introduced based on inspection of a single wafer. For example, a particle defect may be introduced in a photoresist deposition step in a wafer track. Thereafter, the wafer may be patterned using an exposing source via a mask or reticle. The patterned wafer is then coated with a developer solvent to make the exposed pattern either soluble or insoluble according to the type of photoresist employed. The wafer may be optically inspected after the soluble portions of the photoresist have been removed, whereby a defect may be identified. However, without prior inspection of the wafer or other quantization of the defects associated with the initial processing steps, it may be difficult to determine the step at which a defect occurred.
Optical inspection of patterned wafers is typically done using field comparison techniques. The patterning of such wafers commonly includes many fields having theoretically identical component patterns therein. For example, each such field may include a processor or other device. Field comparison techniques presume that where one field in such a patterned wafer is different from the rest of the fields, a significant probability exists that a defect exists in that field. This technique is employed by inspection tools and personnel in quickly identifying likely defects in a patterned wafer.
Although a defect caused in one step (e.g., a photoresist deposition step) may be detectable after multi-step processing, such a defect may be difficult or impossible to differentiate from a defect introduced at a later step, based on a single inspection of the wafer. In characterizing a particular semiconductor processing method, it is desirable to detect a defect in a wafer, and further to identify the process step at which the defect occurred. Accordingly, there remains a need for improved systems and methodologies by which defects in processed wafers, may be identified and located, and by which the causes thereof may be determined. Defect inspections may occur in a blank or unpatterned wafer following an initial processing step, such as a photoresist deposition step. Since the blank wafer has not been patterned, there are no fields. Consequently, field comparison techniques cannot be easily employed to determine the location of defects identified in a blank wafer. Thus, there remains a need for improved systems and methodologies by which defects in a blank wafer may be more easily identified.